There's a lot of things that may have been going wrong.

IE, the vaccine was supposed to be injected into muscle and "stay there". Except medical standard practice of aspirating to ensure they weren't injecting into a vein was abandoned for convenience purposes.

Vaccine storage was rumored to be haphazard. Part of this of course was due to the scale and logistics involved, but it was a multi-layered level of incompetence.

Another problem I think was that there really wasn't a lot of investigation into how physical activity in patients shortly after vaccination might increase risks in a few ways. Possibly increasing the risk of spread from the muscle to the blood, spread of spike proteins, and obviously the potential strain on the cardiovascular system when it's in a somewhat risky state.

And yes, the self-replication part as a whole is a bit iffy. More-so since they built the entire fucking thing off of the spike protein, even though supposedly it's the spike protein that does a lot of the damage.

Just so many stupid mistakes and minimal oversight. Zero responsibility. A total fuck up that's been covered up and buried to high heaven.